The work proposed is an investigation of certain aminoacyl tRNA synthetases in developing red blood cells. Synthetases for histidyl-and possibly methionyl-tRNA will be purified. Particular attention will be paid to multiple forms of these enzymes (isoenzymes) for which evidence already exists. The distribution of the isoenzymes among cellular fractions and the basis of multiplicity in terms of protein structure will be studied. The specificity of multiple forms of the synthetases for tRNA isoaccepting species will be investigated.